a. Field of the Invention
The present invention relates to a method for predicting an imminent episode of ventricular tachyarrhythmia, namely ventricular tachycardia and ventricular fibrillation, based on a rhythm acceleration pattern, and the associated preventive/warning implantable cardiac devices and preventive/warning external cardiac monitoring devices.
b. Description of Prior Art
Currently there is no effective device for predicting, and then preventing, an imminent episode of life threatening ventricular tachyarrhythmia (VTA), namely ventricular tachycardia, which often accelerates into a lethal ventricular fibrillation. Drug therapies can be used, but these anti-arrhythmic drugs typically have undesirable side effects. Clinical experience has shown that at acceptable level, these anti-arrhythmic drugs can reduce but not eliminate completely episodes of VTA. An effective device for treating, as opposed to preventing, ventricular tachyarrhythmia is the implantable cardioverter defibrillator (ICD). An ICD is implanted in a patient at risk of sudden cardiac death, caused by an episode of VTA. However, the ICD can treat, but cannot prevent these episodes of VTA.
In prior art, a number of systems for predicting ventricular tachyarrhythmia have been proposed.
In U.S. Pat. No. 5,425,749 to Adams, et al., issued Jun. 20, 1995, we are taught about a method to give a preemptive shock when an episode of VTA is predicted. One method for prediction is based on detecting a burst of high rate events, typically 3 beats at 200 beats per minute. An alternate prediction uses the “slope filtered pointwise correlation dimension algorithm” which is based on chaos theory measurement of the correlation between samples of R-R intervals. In the applicant's own experience, the first method of prediction is only predictive of a small number of episodes of VTA. The amount of computation involved in the second method for prediction is not easily implemented either in an implantable device or in a bedside monitor.
In U.S. Pat. No. 6,035,233 to Schroeppel, et al., issued Mar. 7, 2000, we are taught about a system that monitors heart rate variability measures such as mean atrial rate, mean absolute deviation, activity, and respiration. A set of normal and abnormal templates are stored. When the measurement changes away from the normal templates towards an abnormal template, a prediction of imminent tachyarrhythmia is made. None of the literature since the filing of this patent has validated the concept of using such templates.
In U.S. Pat. No. 6,058,328 to Levine, et al., issued May 2, 2000, we are taught about preventive pacing to prevent an episode of VTA. The method for triggering this preventive pacing is the deviation of the heart rhythm from a template which can be made adaptive to take into account the history of heart rhythm prior to recorded episodes of VTA. It should be noted that a slow rhythm acceleration would not be considered a marker for VTA for a person skilled in the art at the time of the issue of this patent. Such a rhythm has been considered indicative of a sinus tachycardia in response to increased metabolic demand.
In U.S. Pat. No. 6,115,627 to Street, issued Sep. 5, 2000, we are taught to use the power spectrum to predict an imminent episode of tachyarrhythmia. This is related to the extensive work in the literature on heart rate variability. While it has been recognized that a depressed high frequency (>0.15 Hz) spectrum is a marker for ventricular tachyarrhythmia due to reduced parasympathetic cardiac control, the use of this marker as a predictor of an imminent episode of ventricular tachyarrhythmia has not been documented in the literature since the filing of this patent.
In U.S. Pat. No. 6,161,041 to Stoop, et al., issued Dec. 12, 2000, we are taught about a pacemaker with overdrive pacing to prevent episodes of VTA. The trigger for the overdrive pacing is deviation from stored templates of the QRS and T waveforms, or a coupling time from a QRS complex to a ventricular premature complex that would result in the premature complex occurring during repolarization, i.e. during a T wave. To support these triggers the pacemaker must use non standard sensing circuits since most pacemakers are designed with filters that attenuate T waves.
In U.S. Pat. No. 6,205,357 to Ideker, et al., issued Mar. 20, 2001, we are taught about a system that uses a multitude of electrodes to determine the site of ectopic beats and then deliver preventive therapies in the form of isolated pacing pulses. However, it is well known from the Cardiac Arrhythmia Suppression Trials (Myerburg, et al., “Interpretation of Outcomes of antiarrhythmic clinical trials, Circulation, 1998;97: 1514–1521) that premature (ectopic) beats are not well correlated with sudden cardiac death risk because they happen too often. Thus, the predictor in this system is likely to exhibit a high false positive rate resulting in frequent unnecessary interventions, which themselves may initiate a life threatening ventricular tachyarrhythmia. While these episodes can be terminated by the device, they increase the risk of death by electromechanical dissociation, that may follow an episode of device-terminated ventricular tachyarrhythmia (Mitchell, et al., “Sudden death in patients with implantable cardioverter defibrillators: the importance of post-shock electromechanical dissociation”, J Am Coll Cardiology, 2002:39: 1323–1328).
In U.S. Pat. No. 6,272,377 to Sweeney, et al., issued Aug. 7, 2001, and further refined in U.S. Pat. No. 6,400,982 to Sweeney, et al., issued Jun. 4, 2002, and U.S. Patent Application US 2002/0016550 to Sweeney, et al., published Feb. 7, 2002, we are taught that the implanted device can monitor for marker and trigger conditioning events of ventricular tachyarrhythmia and provide a multitude of preventive therapies including pacing. Example of conditioning events considered are: waveform morphology, specific pattern of activation times of different areas of the heart using a multitude of electrodes in the heart, specific pattern of heart beats with respect to time, heart rate, blood pressure. When a plurality of such conditioning events occurs, the probability of an arrhythmia is calculated and appropriate preventive therapy can be initiated. It should be again noted that a slow cardiac rhythm acceleration would not be considered a marker for VTA for a person skilled in the art at the time of the issue of these Sweeney's patents, or publication of the Sweeney's patent application. Such an acceleration has been considered indicative of a sinus tachycardia in response to increased metabolic demand.
In U.S. Pat. No. 6,308,904 to Shusterman, et al., issued Oct. 23, 2001, we are taught about a system that uses Karhunen Loeve Transformations on the heart rhythm, namely the R-R series, for predicting the occurrences of cardiac arrhythmia from one to three hours prior to the episode of VTA. The computational requirement required for such a system is beyond what can be implemented in an implantable device. Even in a bedside monitoring device, the computational requirement would make this impractical.
In U.S. Pat. No. 6,370,431 to Stoop, et al., issued Apr. 9, 2002, which is a continuation of U.S. Pat. No. 6,161,041 cited above, we are taught to monitor the QT interval and the rate of ventricular extra systoles to predict an imminent episode of VTA. When a prediction of imminent VTA is made, overdrive pacing is initiated. Again, the issue here is that special sensing circuits are needed to detect T waves, which are normally attenuated in conventional pacemakers and ICDs. The rate of ventricular extra systoles has not been found in numerous studies to correlate well with imminent episodes of VTA.
In the ventricular tachyarrhythmia prediction literature, two approaches have been proposed.
Skinner, et al. (Skinner, et al., “A reduction in the correlation dimension of heartbeat intervals preceded imminent ventricular fibrillation in human subjects”, Am. Heart J., 1993:125: 731–743) have proposed a predictor based on a chaos-based measurement. The correlation dimension is the logr (log based r) of the fraction of the total number of heartbeat intervals analyzed within a small radius, r. This type of measurement must be done retrospectively since all the data must be available for analysis. Thus, it cannot be used in real-time to predict an imminent episode of VTA. Note that this algorithm is a more general version of the “slope filtered pointwise correlation dimension algorithm” of U.S. Pat. No. 5,425,749 to Adams cited above.
Mäkikallio, et al. (Mäkikallio, et al., “Heart rate dynamics before spontaneous onset of ventricular fibrillation in patients with healed myocardial infarcts”, Am J Cardiol, 1999:83: 880 884) used a different chaos-based measurement to predict an imminent episode of tachyarrhythmia. The root-mean-square of the deviation of the heart beat intervals, collected over hours, is analyzed using different window lengths. The slope of the deviations over these windows is computed. It was observed that this slope decreases prior to an episode of VTA. The length of data needed to perform a reliable analysis and the complexity of the computation preclude the implementation of this method in a real-time device.
What is needed is a simple and reliable method for predicting an imminent episode of ventricular tachyarrhythmia. Simplicity is needed for implementation in a cardiac device, either an implantable device or a bedside monitor. Simplicity also means that it is easy to adjust to the particular conditions of the patient. In the prior art no simple and reliable method has been described. Furthermore in all the methods described, a prolonged heart rate acceleration, while the rate remains within the accepted normal range of less than 120 beats per minute, would not be considered a marker for ventricular tachyarrhythmia. Such an acceleration would have been considered a normal sinus tachycardia.